MINING IN DOWNEAST MAINE: PROGRESS TOWARD A GEOENVIRONMENTAL MODEL ASSESSING CONSEQUENCES OF MINING METAMORPHOSED MASSIVE SULFIDE DEPOSITS ON THE COAST

The long-term impacts of processes controlling release, transport, and accumulation of metals associated with acid-mine drainage emanating from historic massive sulfide deposits and occurrences in coastal Maine are under investigation. Metal release and transport depend on intrinsic geochemical characteristics of minerals and solids (rocks and ore) and their ability to generate or consume acid. Many of the deposits and occurrences have limited acid-neutralizing capacity because they are associated with felsic metavolcanic and volcaniclastic rocks. Feldspars are altered to clay minerals, further reducing the acid-buffering capacity. Some of the mineralized bodies are also hosted by diverse wall rocks (quartzite, magnesian-rich talc). Carbonates may be abundant locally, thus enhancing the capacity for acid neutralization and promoting precipitation of secondary minerals. Mines and prospects contain >50% sulfides (pyrite, pyrrhotite, chalcopyrite, and sphalerite), silicates (e.g., chlorite, sericite, talc, anthophyllite, tourmaline) and minor carbonates (calcite, dolomite). Accessory sulfides and sulfosalts include enargite, marcasite, galena, arsenopyrite, cobaltite, and tennantite. Weathering produced diverse secondary minerals: e.g., goethite, jarosite, Al-oxyhydroxides, scorodite, halotrichite, and copper-sulfate minerals. The fate of metals released during sulfide oxidation is partly controlled by the nature of the resulting secondary phase (refractory vs. soluble) and by the effect of salinity on solubility. Under certain conditions, biogeochemical controls may also be significant, whereby the release of metals from these secondary mineral phases may be microbially enhanced by indirect reductive dissolution mechanisms, or by direct enzymatic mechanisms. Preliminary geochemical modeling based on mineral reactions indicates that sulfide mineral oxidation, metal transport, and overall reaction rates in mines located in fragile transitional fresh to marine subarctic environments is accelerated or inhibited depending on the particular element and mineral undergoing reaction, compared to mines located farther inland. The exact role of saline fluids and saltwater encroachment remains imperfectly known but periodic saltwater fluxes have the capacity to greatly boost or depress dispersal of metals into the surface environment.